Invertebrate biodiversity

Background

Approximately a million different species of insect are already known - and there are estimated to be around 10 times that number on Earth that are not yet scientifically described. This illustrates the contribution that insects and other invertebrates make to the composition and functioning of all terrestrial ecosystems, including those in urban greenspaces, no matter how small.

In urban areas, invertebrates are often noticed only as pests (such as slugs, aphids or caterpillars) causing visible feeding damage on plant leaves, fruits or seeds in parks and gardens. Yet these relatively few damaging species are only a tiny minority of the invertebrates that make such an important contribution to urban biodiversity. The feeding activities of most invertebrates go unnoticed, but are vital to the functioning of the ecosystem as a whole.

These other invertebrates may feed on tiny bacteria, grains of pollen or fungi. Many are miniature predators, eating other invertebrates and even small vertebrates. Other species are decomposers, feeding on dead wood and leaf litter, carcasses of larger animals, or the soil. As a result of all these activities, flowers are pollinated, the number of pests reduced, unwanted waste broken down, the soil aerated, and nutrients recycled back to the roots of plants. Invertebrates occur at all levels of highly complex food webs, and comprise an enormous biomass that is an important food source for birds and mammals.

A healthy urban greenspace (a domestic front garden, for example) should contain a great variety of invertebrates, among the more familiar of which are beetles, spiders, ants, true bugs (such as shield bugs or aphids), butterflies and moths, flies, wasps, centipedes, millipedes, woodlice, earthworms, nematode worms, mites and springtails. These invertebrates are often present in staggering numbers. For example, around 55,000 springtails (tiny decomposers) were found to be living in a single square metre of soil on a green roof in an urban area of Hanover, Germany (Schrader and Boening, 2006).

Faced with this abundance, maintaining a proper balance in small or disturbed greenspaces requires an understanding of the effects of urbanisation on rare or protected species.

Practical considerations

Urbanisation tends to have a negative effect on biodiversity, primarily due to changes in land management, or the loss or fragmentation of habitats. Indirect effects of urbanisation include pollution and changes in the water balance of soils. Some examples presented here illustrate the impact of urbanisation.

There is considerable diversity of species within many invertebrate groups. For example, there are nearly 4000 species of beetle (Coleoptera) found in the UK, and an estimated 1.5 to 2 million species worldwide (Hammond, 1992). The key to the great success of beetles is essentially the very large range of different habitats to which they have adapted. It follows that a wider range of habitat types will support a greater abundance and diversity of invertebrates.

A wide range of beetle species, both specialist and generalist, can be expected in any urban greenspace, where they will occupy all manner of herbivorous, predatory and decomposer roles. A single tree on a city street will support different beetles in its canopy, on its fruit or seeds, in the moss on its branches, in its hollows, around its roots, in fissures in and beneath its bark, and deeper within any rotting wood. Scaling up to a small urban garden, where habitat diversity increases considerably, additional species will be found in flowers, in the gills of mushrooms, in ants’ nests, in dung, among the roots of a lawn, in grass tussocks, in birds nests, and in compost heaps and stacked wood or old tree stumps. Perhaps most importantly, we find a considerable diversity in the leaf litter and soil of planted borders. If that garden had even a small pond, a whole suite of aquatic and marginal species would also be supported.

Providing a range of different habitat types in an urban environment will promote invertebrate diversity and, in turn, richer communities of plants and higher animals. For example, research has shown that higher diversities of bird species in urban areas are correlated with higher numbers of insect species (Kim et al. 2007).

Forest Research has a team of entomologists with research experience on the sampling, ecology and management of a wide variety of invertebrates. Brief examples of the services offered are given below.

Biodiversity assessments

Sampling invertebrates and assessing their diversity at a local scale is an excellent way of determining the health of an environment. Sampling will ensure that a wide range of taxonomic groups is examined, reflecting their ecological characteristics and function within the habitat. The invertebrate community, or a subset of it, serves as one of the best available indicators of environmental health. Assessments can be carried out using techniques that have negligible impact on the invertebrate community. This might be carried out to form a baseline data set of what is present in the site, which could be used to provide a picture of invertebrate populations before and after a period of development or enhancement work, giving a measure of impact on the biodiversity of the area - be it positive or negative.

Research and consultancy

Forest Research’s services might be used to enhance and better understand biodiversity assessments - such as an ecotoxicology investigation of a brownfield site, or an evaluation of local soil characteristics. Forest Research could offer advice on the most appropriate planting and management programmes for the site to encourage the most valuable vertebrate, invertebrate and plant species for the area.

Case studies

Development of a brownfield site alongside the Thames estuary is currently threatening the only known UK population of a rare ground beetle, the streaked bombadier beetle (Brachinus sclopeta). In the nearby West Thurrock Marshes, a separate development is also threatening a number of rare British invertebrates, including two species of bumblebee and a jumping spider.

Another, perhaps better-known beetle that is also threatened by the urbanisation of its habitat is the stag beetle (Lucanus cervus). This is the largest and most recognisable beetle in the British fauna, the male having greatly enlarged antler-like mandibles, and is a protected species. Despite its fearsome appearance, the adults feed by licking tree sap, and breed in rotting wood. Its distribution in Wales and southern England has greatly declined, largely due to the reduction of its breeding habitat through tidying up and removing tree stumps in gardens, parks and woodlands. It is still locally common in south-east England, including London, and survey and conservation work in this region aims to promote the maintenance of these populations. This can be achieved by leaving broadleaf tree stumps in situ and providing dead-wood habitats where possible. As a recognisable flagship species, this important invertebrate highlights how a range of habitat types in any greenspace area will promote biodiversity.

A study of the herbivorous insects living on the vegetation in urban brownfield sites in northern Germany found that the species present at a site varied according to the successional stage of the vegetation. This meant that a different suite of species were found in open, low vegetation, compared with denser, taller vegetation (as found in more established habitats). The research concluded that to maintain the highest diversity of species in an area required a habitat mosaic of different successional stages (Strauss and Biedermann, 2006).

Another study of urban invertebrate diversity sampled Hemiptera (true bugs) on vegetation-covered roundabouts of different sizes in Bracknell, south-east England. As might be expected, larger roundabouts were found to support a greater species diversity - but it was also found that intensive management (frequent mowing of grassland areas) counteracted size effects and reduced Hemipteran species diversity. The choice of trees planted was also seen to affect insect biodiversity in these greenspaces (Helden and Leather, 2004). A similar study in Finland found that the timing of mowing played an important role in promoting butterfly and moth (Lepidoptera) diversity in roadside verges. By delaying mowing until the end of summer, or leaving a mosaic of mown and unmown areas, more species were able to complete their life cycles unhindered, and adults were able to find food in the form of flower nectar when they needed it (Valtonen et al., 2006).

Perhaps one of the most important terrestrial invertebrate communities, and yet often the least well studied and understood, is that in the soil and leaf litter. An investigation into the soil invertebrates across a series of urban parks and gardens in London examined the biodiversity of two key habitats: mown grass lawns and plant borders (flower beds). Most of the species collected were commonly associated with disturbed or cultivated areas, but overall species diversity was found to be comparable to the diversity of soil invertebrates in a native woodland sampled in Hampshire. Plant borders were found to contain significantly more species than lawns, and in the borders where leaf litter was most abundant (not cleared away), species diversity was greatest. Leaf litter not only provides a food resource for decomposers such as earthworms and millipedes, but helps to maintain soil humidity and create a broader range of habitats (Smith et al., 2006).

As urban greenspaces are often small, these studies illuminate the importance of management in promoting invertebrate diversity. Provision of a range of habitat types can be achieved by simple alteration of management practices, such as less regular mowing of grassy spaces, and less intensive clearing away of leaf litter and dead wood.